Jupiter at opposition in September night sky

Monday 30 August 2010 03.01 EDT
First published on Monday 30 August 2010 03.01 EDT

Three stars rule our high S sky tonight, while three planets are at their best during September. The stars, Vega, Deneb and Altair, form our Summer Triangle which topples westwards overnight until Altair sets in the W before dawn.

The planets are Mercury, Jupiter and Uranus. Jupiter, indeed, is marginally closer and brighter at opposition in Pisces on the 21st than it has been for 47 years. It is then 592m km away and conspicuous at mag -2.9 as it climbs from our E horizon at nightfall to pass about 35° high in the S at 01:00 BST. Jupiter shows a large 50 arcsec disc through a telescope, while binoculars reveal its four main moons. Look for it near the Harvest Moon on the 22nd and 23rd.

Jupiter also appears close to the distant planet Uranus which is another easy binocular object of mag 5.7. As both creep westwards, Uranus shifts from 1.8° due W (right) of Jupiter tonight to lie 0.8° (less than two Moon-breadths) NNW of Jupiter on the 19th and 1.4° NE of Jupiter by the month's end. Uranus is 2,856m km wide at opposition, its blue-green disc only 4 arcsec wide. Mercury begins its best morning apparition of the year as it emerges from the Sun's glare late next week. From the 13th to the month's end, it rises more than 80 minutes before the Sun, stands between 7° and 10° high in the E 30 minutes before sunrise, and brightens from mag 1.1 to -1.1. Check our next Starwatch for more details.

It is hard to be excited about the other naked-eye planets this month. True, Venus is brilliant at mag -4.4 but it stands a mere 6° high in the SW at sunset on the 1st and will soon be lost as it sinks even lower. Mars and Saturn, and Virgo's star Spica, are not far away but already swamped by the evening twilight, at least as seen from our N latitudes.